It is known to conduct the video IF carrier and the audio IF carrier of a television receiver via a common IF amplifier in which the gain for the audio carrier is reduced compared to that for the video carrier. At the output of the video rectifier, which operates as an amplitude demodulator, there then appears, inter alia, the difference frequency of 5.5 MHz between the video IF carrier and the audio IF carrier which is fed to the FM audio demodulator via a further amplifier. In this intercarrier circuit, the audio reproduction is relatively sensitive to interference by the video signal.
In parallel sound television receivers, the video IF carrier and the audio IF carrier are transmitted via separate amplification paths to separate demodulators so that they cannot influence one another and mutual interference between video and audio can no longer occur. However, such a circuit entails additional expenditures.
It is also know, as disclosed in German Pat. No. 1,138,813, to selectively couple the audio IF carrier and the video IF carrier out of the IF amplifier, to free the video IF carrier from the modulation by the video signal and then to obtain an inter carrier of 5.5 MHz by mixing the two carriers. The elimination of the modulation which represents the picture content is then essentially effected by amplitude limitation of the video IF carrier. However, in this circuit there still remain phase fluctuations which are dependent on the picture content in the mixed carrier obtained by removing the picture dependent modulation and these phase fluctuations may also lead to interference in the audio reproduction.
It would also be conceivable to generate the mixed carrier in an oscillator which oscillates freely at precisely the video IF and then to mix it with the modulated audio IF carrier to obtain the intercarrier. Then the mixed carrier would be free of any amplitude or phase modulation. But this solution cannot be realized in practice for the following reason:
Microphony phenomena and electrical stray currents from the mains or the vertical deflection member cause slight changes in frequency in the oscillator of an order of magnitude of several hundred hertz so that the frequencies of the video IF carrier and of the audio IF carrier change in the same way. Since the difference carrier, or intercarrier, is obtained by mixing these two carriers, these changes in frequency have practically no effect in the resulting intercarrier. However, if the audio IF carrier is subjected to these frequency fluctuations but the mixed carrier is not, the resulting audio will again be subject to interference from changes in the oscillator frequency.